With the rapid development of the electronic information industry, a chip manufacturing industry moves into a nanometer times. A chip manufacturing process size is reduced from 90 nm to 45 nm, and then to 30 nm and 13 nm, which is quickly progressed. The physical dimension of the chip is more and more small, which is from 1.0 mm×1.0 mm to 0.8 mm×0.8 mm, 0.5 mm×0.5 mm, 0.3 mm×0.3 mm, and minimum 0.15 mm×0.15 mm. Accordingly, the pitch of the pad used for chip manufacturing is gradually reduced from 120 μm to 100 μm, 70 μm, 60 μm, 50 μm and 45 μm. and a scribing channel is also gradually reduced from 100 μm to 70 μm, 60 μm, 504 μm and 45 μm. Therefore, the size of the pad is also gradually reduced from 100 μm×100 μm originally to 70 μm×70 μm, 55 μm×55 μm, and minimum 38 μm×38 μm. The size change of the pad brings difficulty and great challenge for the bonding process.
In a normal ball welding binding process, the qualified diameter of a welded ball is larger than/equal to 2 times of a wire diameter, and smaller than 5 times of the wire diameter. Generally, the diameter of a metal wire welded ball can be controlled at 2˜2.3 (times) of the wire diameter, and the diameter of a copper wire welded ball can be controlled at 2.5˜3 (times) of the wire diameter. For 38 μm×38 μm pads, the pitch of the adjacent pads is 41 μm˜43 μm, and a phi 15 μm˜phi 16 μm welding wire can be used only; moreover, the diameter of the bonding welded ball must be controlled within 37 μm. In this way, whether the phi 15 μm (2.3×15=34.5<37) metal wire or the upper limit phi 16 μm (16×2.3=36.8<37) metal wire can meet the pressure welding requirements. While whether the phi 15 μm (taking the lower limit 15×2.5=37.5>37), or the phi 16 μm (taking the upper limit 16×2.5=40>37) copper wire does not meet the pressure welding requirements. Moreover, from the view of pressure welding quality inspection, the clearance between adjacent bonding wires shall be equal to 2 times of the diameter of the welding wire. Actually, the distance between the two adjacent bonding wires cannot meet general quality requirements even if the wire diameter is removed and the clearance between the adjacent bonding wires is 1.69˜1.86 times of the wire diameter. However, practice proves that with the improvement of the quality of the welding wire, increasing of wire diameter specifications, development of packaging technology and increasing of high density packaging forms and products, under the premise of ensuring that the plastic packaging punching wire rate meets the technical requirement, it is also accepted to meet the welding wire technical requirement by the industry when no short circuit exists between the welding wires and the clearance between adjacent leaders is larger than 1 times of the wire diameter. However, for dense-pitch small-pad bonding, it is very difficult to control the diameter of a bonding wall. A little carelessness may make the bonding ball exceed the pad to cause short circuit between adjacent welding spots and lead to product abandon. Moreover, since the wire spacing is too small, a previous wire will be hit when a second wire is punched to cause damage of the previous wire. So, the biggest difficulty for bonding a dense-pitch small-pad product having a pitch less than 43 μm (38 μm×38 μm) lies in the problems of controlling the diameter of the welded ball at the bonding spot and damaging the adjacent welding wires by hit.